1. Field of the Invention
This invention relates to signal detecting circuits, and is more particularly directed to a signal detecting circuit for detecting whether an input signal contains a component of a predetermined frequency.
2. Brief Description of the Prior Art
It is the general practice to use a spectrum analyzer to determine what frequency components exist in an input signal. However, a spectrum analyzer is a complex and expensive piece of electronic equipment. Thus, when it is necessary only to determine whether a single particular sinusoidal component is present in an input signal at a particular time, it is preferable to use a simpler, less expensive arrangement than the spectrum analyzer.
For example, in a color television receiver, it is often desirable to determine whether a color subcarrier is present in an input video signal received thereby, for instance, to control a color killer circuit, or to control a luminance/chrominance separator circuit.